--interface=interface Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback), which may turn out to be, for example, ``eth0''.

$tcpdump -i eth0

-c count

Exit after receiving count packets.

$tcpdump -c 30 -i eth0

-D

--list-interfaces, prints the list of the network interfaces available on the system and on which tcpdump can capture packets. For each network interface, a number and an interface name, possibly followed by a text description of the interface, is printed. The interface name or the number can be supplied to the -i flag to specify an interface on which to capture.

$tcpdump -D

-w file

Write the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option.